HTTP Response Splitting

Last reviewed: September 4, 2026

HTTP Response Splitting is a web application vulnerability that arises when an attacker is able to manipulate the HTTP headers of a web server's response. This manipulation can lead to various attacks, including web cache poisoning and cross-site scripting (XSS). The vulnerability occurs when user input is included in HTTP headers without proper validation or sanitization, allowing an attacker to inject malicious content. As of October 2023, HTTP Response Splitting remains a concern for web developers and security professionals due to its potential impact on web application integrity and user data security.

Overview

HTTP Response Splitting is a type of web application vulnerability that exploits improper handling of input data in HTTP headers. This vulnerability allows an attacker to inject arbitrary HTTP headers or content into a server's response. The attack is possible when user input is directly included in HTTP headers without adequate validation or sanitization. This can lead to various malicious activities, such as web cache poisoning, cross-site scripting (XSS), and session fixation. The vulnerability is particularly concerning for web applications that rely on caching mechanisms, as it can affect multiple users.

How it works

HTTP Response Splitting occurs when an attacker is able to insert CR (Carriage Return) and LF (Line Feed) characters into HTTP headers. These characters are used to delineate headers and separate them from the body of the HTTP response. By injecting these characters, an attacker can create additional headers or manipulate the response body.

Input Manipulation: The attacker identifies a point in the web application where user input is included in HTTP headers. This could be a URL parameter, a form field, or any other input that is reflected in the server's response.

Injection: The attacker crafts input that includes CR and LF characters, followed by malicious headers or content. For example, an input like `%0D%0ASet-Cookie:malicious=true` could be used to inject a new cookie into the response.

Response Splitting: The server processes the input and includes it in the HTTP response headers. The CR and LF characters cause the server to interpret the input as new headers, effectively splitting the response.

Exploitation: The attacker can exploit the manipulated response to perform various attacks, such as:

Web Cache Poisoning: By injecting headers that affect caching behavior, the attacker can poison the cache with malicious content.
Cross-Site Scripting (XSS): Injected scripts can be executed in the context of the victim's browser.
Session Fixation: The attacker can set session cookies to hijack user sessions.

Observed use

HTTP Response Splitting has been observed in various real-world attacks, often targeting web applications with inadequate input validation. Attackers have used this vulnerability to poison web caches, to the distribution of malicious content to multiple users. Additionally, HTTP Response Splitting has been leveraged to execute cross-site scripting attacks, allowing attackers to steal sensitive information such as session cookies and user credentials.

The vulnerability has been reported in several high-profile incidents, highlighting the importance of proper input validation and sanitization in web applications. Security researchers have demonstrated the feasibility of HTTP Response Splitting attacks in controlled environments, emphasizing the need for awareness and mitigation strategies among developers and security professionals.

Detection

Detecting HTTP Response Splitting vulnerabilities requires a combination of manual testing and automated tools. Security professionals can use the following methods to identify potential vulnerabilities:

Code Review: Conduct a thorough review of the web application's code to identify areas where user input is included in HTTP headers. Look for instances where input is not properly validated or sanitized.

Automated Scanning: Use automated security scanners that are capable of detecting HTTP Response Splitting vulnerabilities. These tools can simulate attacks and identify potential weaknesses in the application.

Penetration Testing: Perform penetration testing to simulate real-world attacks and identify vulnerabilities. Testers can craft malicious input to see if the application is susceptible to HTTP Response Splitting.

Monitoring and Logging: Implement monitoring and logging mechanisms to detect unusual patterns in HTTP responses. Anomalies in response headers or unexpected behavior can indicate a potential attack.

Mitigation

Mitigating HTTP Response Splitting vulnerabilities involves implementing robust input validation and sanitization practices. The following strategies can help prevent this type of attack:

Input Validation: Validate all user input to ensure it conforms to expected formats. Reject input containing CR and LF characters or other potentially malicious content.

Output Encoding: Encode user input before including it in HTTP headers. This prevents the injection of special characters that could be used to manipulate the response.

Use of Frameworks: Utilize web frameworks that automatically handle input validation and sanitization. These frameworks often include built-in protections against common web vulnerabilities.

Security Headers: Implement security headers such as Content Security Policy (CSP) and X-Content-Type-Options to reduce the impact of potential attacks.

Regular Security Audits: Conduct regular security audits and vulnerability assessments to identify and address potential weaknesses in the application.